Amplifier circuit arrangement



Augf.`28,1945. T. H. PRICEV- 2,888,888

, AMPLIFIER CIRCUIT ARRANGEMENTl Filed Feb.' 2, 1942 2 Sheets-Sheet 2 1 Travail: fier- Il T INVENTOR 72/0/:446 HENRY 10e/c5 ATTORNEY combined outputs to Patented ug. 28, 1945 AMPLIFIER CIRCUIT ARRANGEMENT Thomas Henry Price, Chelmsford, England, assignor to Radio Corporation of America, a' corporation of Delaware f Application February 2, 1942, Serial No. 429,144

, InGreat Britain March 13, 1941 Claims. (Cl. 179-171) The present invention relates to amplifier cirlcuit arrangements and provides an improved feed-back arrangement. The invention is particularly applicable to radio-telephone transmitters where it may serve for the reduction of hum caused by heating the valve filaments by alternating current or by insuflicient smoothing of the anode supply. It is not, however, so limited.

As an example of the kind of difliculty which the invention may be employed to reduce or overcome, the following may be considered. One of the principal difiiculties to be encountered in the application of high level negative feedback to several amplifying stages in cascade is that phase shift occurs and this increases with frequency, until at some relatively high frequency the whole system in the feedback loop oscillates or-sings. There is also very often a tendency for the system to oscillate at some very lowv frequency. Either one or the other of these oscillations invariably sets in before some prescribed level of feedback can be achieved. The invention provides means which postpone th'e onset of these oscillations until the level of feedback shall be the value to be adopted.

According to the invention, an amplifier system comprises at least a first and a second amplifierstage in cascade, a source of modulating-signal energy, a first and a second lter, a balancing circuit arrangement having two input circuits,

Ia voltage divider, associated with said source of modulating-signal energy, and including means' for applying a portion of modulating-signal energy from said source to the input of said first amplifier stage, means for applying modulatingsignal energy from said source to one-input circuit of said balancing circuit arrangement through said first filter, means for extracting modulation signal from the output of said second amplifier stage, means for applying said extracted modulation signal to the other input circuit of said balancing circuit arrangement through said greater than two input circuits, a voltage divider associated with said source of modulating-signal energy and including meansl for applying a portion of said modulating-signal energy from said source to one input circuit of said first amplifier stage,

means for applying modulating signal. energy from said source to one input circuit of said I balancing-circuit arrangement, means for extracting modulation signal from the output of said .second amplifier stage, means for applying said extracted modulation signal to the other input circuit of said balancing-circuit arrangement, means for applying the output from said balancing-circuit arrangement t0 another input circuit of said rst amplifier stage and means including said rst amplifier stage for combining in correct phase for the elimination of distortion f the output o f said balancing-circuit arrangement with said portion of modulating-signal energy applied to said one input circuit of said first ainplie;` stage, and means for applying the energies combined in said combining means to the input of said second amplifier stage.

According to a further feature of theinvention, an amplifier system comprises at least a first and a second amplifier stage incascade, a source of modulating-signal energy, means for applying modulating-signal energy from said source to said first amplifier stage, a first and a second second filter, and means including said rst ampliiier stage for combining in correct phase for the elimination of distortion the output of said balancing circuit arrangement-with the output of said first amplifier' stage and for applying the the input of said' second amplifier stage.

According to a feature of the invention.- an amplifier system comprises at least a first amplifier having two input circuits and a second amplifier-stage in cascade, a source of modulatingsignal energy, a rst and a second filter consti tuted al a balancing circuit arrangement having filter constituted as a balancing-circuit arrangement having two input circuits, a voltage divider associated with the output circuit of said first amplifier stage and including means for applying a portion of the amplified modulating-signal energ'y to one input circuit of said balancing-circuit arrangement, means for extracting modulation l signal from the output of said second amplifier stage, means for applying said extracted modulation signal to the other input circuit of said balancing-circuit arrangement, combining means including' said first amplifier, for applying the output from said balancing-circuit arrangement to the input circuit of said first Iamplifier stage for combining in correct phase for the elimination of distortion the output of said balancingcircuit arrangement with the modulating-signal energy applied to the input circuit of said first amplifier stage, and means for applying energies combined in said combining' means to the input of said second amplifier stage.

If. there are more thantwocascaded amplifier stages, the modulation signal may be extracted from the final amplifier stag/e. If the output of the second or 'final amplifier stage from which the modulation signal is extracted be modulated Irrier, a demodulator would be included between the output circuit of said second or final amplifier stage and vsaid, second filter.

The balancing circuit may include a thermionic valve having at least two grid-electrodes, the modulating-signal energy being applied through said first filter to one of said grid-electrodes and the extracted modulation signal being applied through said second filter to another of said grid electrodes.

The combining means may include the first amplifier stage and may comprise a valve amplifier having at least a control grid and an anode, lthe signals to be combined being applied, respectively to the control grid and anode; or it may comprise a valve amplifier having at least a control grid, both signals to be combined being applied to the said control grid'; or it may comprise a valve ampliiier having at least two grids, the signals to be combined being applied one to one grid and the other to the other; or it may comprise two valve amplifiers each having at least one control grid, the signals to be combined being applied one to the control grid of one valve amplifier andthe other to the control grid of the other valve amplifier.

The two filters and the balancing circuit arrangement may take other formsefor instance capacity-impedance (including capacity-inductance-resistance combinations) and their equivalents, for example piezo-electric crystal circuits.

The voltage divider may comprisea potentiometer.

' The invention is illustrated in the accompanying drawings of which Figure l is a block diagram illustrating the basic principle of the invention,.Figure 2 is a circuit diagram of the arrangement of Figure l, Figures 3 and 5 are circuit diagramsof modified arrangements, and Figure 4 is a circui-t diagram illustrating the application of the arrangement of Figure 3 to a transmitter.

In the embodiment of the invention illustrated in Figure l and as applied to a two stage cascaded amplifier arrangement, audio-frequency modulating-signal energy is applied over a pair of conductors IP to the two ends of a potentiometer resistor ri and to the input end of a first filter network Fl which may comprise resistors and condensers. A portion of the modulatingsignal energy, derived from an adjustable tapping point pl on the potentiometer resistance, is ap-- plied to the input circuit of a first amplifier stage AI. The output of the first amplifier stage is applied to the input circuit of a second amplifier stage A2, and the output of this second amplifier stage is applied to a suitable load circuit which, for convenience may be regarded as a resistor r2. Amplified modulating-signal energy appears across this resistor. By means of an adjustable tapping point p2 on this resistor a portion of the modulating-signal energy, of a desired amplitude, is applied to the input end of a second filter network F2, which, like the first filter network, may comprise resistors and condensers, and which has the same characteristic as the rst filter network. The output of the first lter network Fi is applied between one grid and the cathode of an amplifier (not shown) and the output of the second lter network F2 is applied between another grid and the cathode of this amplifier, the amplifier constituting a balancing circuit BS. The balancing circuit BS must, of course, satisfy the requirement that zero output be obtained escasas for all amplitudes and opposite phases applied to the two filters. The output of BS is applied to the first amplifier stage Ai. l A detector D may be required, as will be seen ater.

In the circuit arrangement illustrated in Figure 2, modulating-signal energy is applied, by means of a transformer t to the gridfcathode space of a valve amplifier VA which has, in its anode circuit, a series of resistors including potentiometer resistor 1|. The output of the valve amplifier VA is fed to the first filter FI which includes resistor RI and condenser CI in series in the anode/cathode circuit of valve-amplifier VA and condenser C2 and resistor R2 in series across condenser Ci. The `function point of C2 and R2, and the junc- -tion point of Ci and R2 constitute the output terminals of the rst filter Fl.

(It should here be observed, that stage AI, though not necessarily the first amplifier stage in the transmitter considered as a whole, is the first amplifier stage with which the invention is directly concerned.)

The output of filter Fi is applied between the grid and cathode of one valve VI of a balancing circuit BS.

A portion of the modulating-signal energy, derived from an adjustable tapping point pi on the potentiometer resistance, is applied to the input circuit of a first amplifier stage Ai. The output oi the first amplifier stage is applied to the input circuit of a second amplifier stage A2, and

' the output of this second amplifier stage is applied throughv further stages to a resistor r2. Amplified modulating-signal energs1 appears across this resistor. By means of an adjustable tapping point p2 on this resistor a portion of the modulating-signal energy, of a desired amplitude, is applied to the input end of a second filter network F2 which is constructed like the first filter networkand includes resistor Rand condenser C3 in series and condenser Cd and resistor R45 in series across condenser C3. The junction point of Cd and R6, and the junction point of C3 and Ell constitute the output terminals of filter network F2. The output of filter F2 is applied between the grid and cathode of the other valve V2 of the balancing circuit BS.

The anodes of valves Vi and V2 areconnected together through resistor r3 and, by tapping point P3 to the anode of valve V3 which constitutes the first amplifier stage and also the means for combining the forward and backward fed energies. As

l the anodes are joined together, and to the anode of valve V3, the only output from Vi and V2, which joins with the output of valve V3, is the difference between the forward and backward fed signals of valves VI and V2. A

If there be hum or spurious effects on the backward fed signal, this is amplified and passed on with the output of valve V3 to give the desired suppression of such h'um or spurious effects. The simple and limited filtering obtained with a C and R combination has the advantage that the maximum phase shift which they produce cannot exceed whereas many types of filters produce greater phase shift, Filters or frequency discrirninators having no phase shift when their filtering comes on would be the ideal, but this is impossible with present day types. The values of the resistance and capacity must be so chosen that the phase shift for the forward signal fed fcllrwardii. the girid of valve Vi is equal to the p ase s expe enced by the si al fe the grid of valve V2. gn d back to aeeaeae ancingcircuit arrangement. It will be seen that the two resistors R2 and Rl, and the two valves VI and V2, of Figure 2 are replaced by a common resistor R24. Figure 3 shows, also a modified first amplifier stage AI wherein two valves V3I and V32 replace valve V3 of Figure 2. The portion of the modulating-signal energy which, in Figure 2, is applied to the grid of valve V3 is, in Figure 3, applied to the grid ci' valve V32, and the output from the balancing circuit, which in Figure 2, is obtained from pointl p3 on resistor r3 and is applied to the anode of valve V3 is, in Figure 3 obtained from point P3 and is applied to the grid of valve V3i. As, in Figure 2, valve V3 served for combining or mixing the forward and backward fed energies, so, in Figure 3, valves V3i and V32, or rather theircommon output, serve this purpose. Apart from these differences the arrangement of Figure 3 is similar to that of Figure 2.

Figure 4, which, in all essentials connected with the invention is identical with Figure 3, indicates, that where the output from amplifier stage AI is applied to a radio (or like carrier) Vtransmitter TR, a demodulator, indicated at D,

is fed with modulated-carrier energy where modulation signal for feed back purposes is extracted and applied to resistor r2.

i Figure illustrates a further modification. A study of Figure 4 shows that for a high level of feedback, theuseful portion of the forward input signal applied to V32 is only a part of the signal developed by the first valve. But a high level of signal is required for the purpose of effecting high level feedback. In the arrangement of Figure 4 there is a sacrifice of gain, which may or may not be important.

The arrangement of Figure 5 effects an improvement in that there is no sacrifice-of gain. Valves V3 and V3, shown as pentode valves, but

which may be other types, amplify the input signal, and the output from V4 is passed to the second amplifier stage of the transmitter.V The demodulator D receives a modulated signal from the transmitter and the extracted modulation signal of the correct phase is obtained from either resistor r6 or resistor f5 by means' of switch S2. 'Ihis signal is applied to the rebalancing circuit. As, however,lthe signal energy is so distorted, the distortion signal appears in the output of the balancing circuit and is fed back to the input of the amplifier, where it is sistance R3 of the filter F2 which as in Figure 3 forms part of the balancing circuit. The components of the filters Fl and F2 are so chosen that, for the required level of feedback, over some given modulation frequency band, no oscillation around the amplifiers and transmitter can occur. The tapping point pl on resistor rl, connected across the output of valve VQ, is adjusted so that no change of amplification of the system occurs with switch SI open or closed; that is, the feedback is operative on only the spurious effects arising in the transmitter. The output from the filters and combining circuit is fed by way of transformer t from P3 whence it is applied, together with the modulating signal energy, to the c grid of valve V3, constituting with valve V4 the first amplier stage, and also serving as thev mixer. It will be seen that the rst amplifier stage, in this arrangement precedes the lter and balancing circuit.

The amplitude and phase of the signals applied to the' balancing circuit are so adjusted that, were there no distortion of the modulating-signal energy as it passed through the amplifler stages, there would be no output from the `amplified (in valves V3 and V4) together with the normal modulation signal and passed on to A2. The system thus provides a high-level negative feed-back for the elimination or reduction of hum and distortion generally.

It may be observed that without the filters it would be impossible to effect high level feed-back lby balancing the forward and feedback signal in a balancing stage or network. 'I'he reason for this is that when a number of valve amplifiers are'connected in cascade phase shifts of the modulating signals occur in traversing the amplifiers, the phase shift being at its worst and constituting a major difliculty for frequencies below the lower limit of audio frequencies, and again beyond some relatively high value of modulating frequency or its harmonics. The result l is, that at these frequencies a practical balance out the filters, and also to provide sufficient attenuation to modulating, and harmonics of modulating, frequencies which experience such phase shifts in traversing the system that they would in theabsence of filters produce undesirable effects on the frequency response and/or the distortion of the modulation signal in the output of thetransmitter.

The invention is the result of considering the requirement in a complete transmitter, particularly with alternating current lighting of the filaments with the consequential needof high level feed-back to bring the hum down tolacceptable values. of several audio frequency stages and one or more stages of 'radio frequency amplification, the phase shift problem of the transmitter itself,

and the high level of feedback required with f alternating current filament lighting, together with the fact that present day filters themselves introduce some phase shift before providing great attenuation, actually results in the condition that the high level of feedback obtains only over-the lower end of the audio frequency spectrum. But with simple filter networks consisting of resistance and capacity it has been demonstrated that between 30 and 40 decibels of feedback over the range of hum frequencies can be obtained when the filament energising'is with 50 cycles per second current. If the level of feedback be allowed to be lower, a suitable adjustment of the filter components will effect a widerfrequency range of feedback to deal with distortion over a wider "band of modulating frequencies.

With a transmitter consisting ond loop may be used, incorporating fewer stages and preferably (though not necessarily) the later stagesand a feed-back circuit as above described so designed as to give a wider eective frequency band of feed-bach.` With such an arrangement the necessary eiective feed-back for the provision of a carrier substantially free from hum and a modulation of high iidelity is obtained. The Width of frequency band .of feed-back may of course be controlled by suitable choice of the electrical dimensions of the elements of the filters.

Having now particularly described and ascertained the nature of my said invention and'in what manner the same is to be performed I decl'are that :what I claim is:

1. An amplifier system comprising at least a iirst and a second amplifier stage in cascade. a source of modulating-signal energy, a iirst and a second filter, a balancing circuit arrangement having two input circuits, a voltage divider associated with said source of modulating-signal energy and including means for applying a portion of modulating-signal energy from said source to the input of said rst amplifier stage, means for applying modulating-signal energy from said source to one input circuit of said ba ancing circuit arrangement through said rst A iilter, means for extracting modulation signal from the output of said second ampliiier stage, means for applying said extracted modulation signal to the other input circuit of said balancing circuit arrangement through said second nlter, and means including said first amplier stage for combining in correct phase for the elimination of distortion the output of said balancing circuit arrangement with the output of said rst amplier stage and for applying the combined outputs to the input of said second ampliiier stage.

2. An amplier system comprising at least a iii-st amplifier having two input circuits and a second amplifier stage in cascade, a source of modulating-signal energy, a first and a second filter constituted as a balancing-circuit arrangement having two input circuits, a voltage divider associated with said source of modulating-signal energy and including means for applying a pori tion of said modulating-signal energy from said source to one input circuit of said rst ampliiler stage, means for applying modulating signal energy from said source to-one input circuit of said balancing-circuit arrangement, means for extracting modulation signal from the output of said second amplifier stage, means for applying said extracted modulation signal to the other input circuit ofsaid balancing-circuit arrangement, means for applying the output from said balancing-circuit arrangement to another input Bti asaaase circuit of said first amplier stage and means including said rst amplier stage for combining in correct phase for the elimination of distortion the output of said balancing-circuit arrangement with said portion of modulating-signal energy applied to said one input circuit -of said rst ampliiier stage, and means for applying the energies combined in said combining means to the input of said second ampliiier stage.

3. An amplifier system comprising 'at least a rst and a second amplifier stage in cascade, a source of modulating-signal energy, means for applying modulating signal energy from said source to said iirst amplifier stage, a first and a second illter constituted as a, balancing circuit arrangement having two input circuits, a voltage divider associated with the output circuit of said iirst amplifier stage and including means for ap plying a portion of the ampliiied modulatingsignal energy to one input circuit of said balancing-circuit arrangement, means for extracting modulation signal from the output of said second amplifier stage, means for applying said extracted modulation signal to the other input circuit of said balancing circuit arrangement, combining means including said rst amplifier for applying the output from said balancing-circuit arrangement to the input circuit of said first ampliiier stage for combining in correct phase for the elimination of distortion the output of said balancing-circuit arrangement with the modulating-signal energy applied to the input circuit o said first amplifier stage, and means for applying energies combined in'said combining means to the input of said second amplier stage.

4. A method of amplifying signals to eliminate distortion which includes amplifying the signals, subjecting a portion of the signals to separate amplification, phase shifting the separately amplified signals, combining the phase shifted signais with the first amplied signals, deriving a portion of the combined signals', and combining the last portion withthe separately amplied I signals.

5. ,A method of amplifying signals without distortion lwhich includes the steps of subjecting the A original signals to a predetermined phase shift, subjecting a predetermined portion of the original signals to a separate phase shii't equal to the iirst phase shift, amplifying the two phase shifted signals in such relative amplitude and phases that in the absence of distortion of the, original-4 signals the output is zero, and balancing the lastnamed amplied signals against the said predetermined portion of the signals thereby to eliminate any distortion in the original signals.

THOMAS HENRY PMCE. 

